Experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, is induced by generating T-cell mediated immunity to central nervous system antigens. The primary autoantigen in EAE in B10.PL mice is myelin basic protein (MBP). The immune response to MBP is predominantly directed toward residues MBP 1-11, with minor responses detected to MBP 31-50 and MBP 121-140. The basis for the dominance of MBP 1-11 and the extent to which tolerance mechanisms influence immunodominance are not understood. The role played by subdominant epitopes in autoimmune disease is also not well characterized. The extent to which subdominant epitopes participate in determinant spreading in the later stages of autoimmune disease may depend on the mechanisms responsible for their subdominance. Preliminary experiments show that endogenous expression of MBP significantly influences immunodominance of particular epitopes. In mice lacking endogenous MBP (MBP-/-), the strongest immune response is now directed toward MBP 121-140, with MBP 1-11 responses comparable to those observed in wild-type mice. Thus, wild-type mice induce tolerance in most MBP 121-140 specific T-cells while MBP 1-11 specific T-cells escape tolerance. The proposed research investigates the mechanisms underlying this differential tolerance. The investigators' hypothesis is that interactions between MBP 121-140/I-Au complexes and T-cells are of sufficient avidity to induce tolerance by clonal deletion, while interactions between MBP 1-11/I-Au complexes and T-cells are lower avidity and less able to induce tolerance. To test this hypothesis, kinetic experiments to compare both the binding affinities of MBP 1-11 and 121-140 for I-Au MHC protein as well as the effective affinities of MBP 1-11 and 121-140 specific T-cells for their respective ligands will be performed. Two possibilities for incomplete tolerance to MBP 121-140 will be addressed using both kinetic experiments and a comparison of T-cell receptor repertoires in MBP 121-140 specific T-cells from MBP+/+ ("wild-type") and MBP -/- mice: 1) the minor response in wild-type mice consists of clones that escaped tolerance induction because they express lower affinity T-cell receptors than those found in MBP -/- mice, or 2) some MBP 121-140 specific T-cells in wild type mice randomly escape tolerance mechanisms and are few in number but still highly reactive to antigen. T-cell receptor transgenic models will also be developed to define the mechanisms and the site of tolerance induction to MBP 121-140.